1. Field of the Invention
The present invention relates to a lithographic projection apparatus.
2. Description of the Related Art
The term xe2x80x9cpatterning devicexe2x80x9d as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a pattering device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a pattering device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clens.xe2x80x9d However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791.
An important determinant of imaging quality in a lithographic apparatus is uniformity of the dose delivered to the substrate surface. If there are variations in the energy density delivered at wafer level across the imaged area this can lead to variations in the size of image features after development of the resist. Uniformity at wafer level can be ensured to a high degree by ensuring that the illumination field (slit) at mask level is uniformly illuminated. This can be achieved by passing the illumination beam through an integrator such as a quartz rod, within which the beam will undergo multiple reflections, or a fly""s eye lens, which creates a multiplicity of overlapping images of the source. A fly""s eye lens, or its equivalent, can be made in both refractive and reflective optics but may still leave some residual non-uniformity in intensity across the illumination field.
U.S. Pat. Nos. 6,013,401 and 5,895,737 describe step-and-scan apparatus having an arrangement for controlling the illumination intensity along the length of a rectangular illumination field that is perpendicular to the scanning direction. The device comprises a plurality of linked blades arrayed along one edge of the illumination field or slit. The blades are selectively inserted into the slit to reduce its effective width so that the effective slit width can be varied along its length. By reducing the effective slit width at positions where the illumination energy density is relatively high, a more uniform illumination of the mask and hence a more uniform dose at substrate level can be obtained while scanning. However, this arrangement introduces telecentricity problems because it causes asymmetric filling of the pupil of the projection system and shifts the center of gravity of the illumination beam. Also, the sliding mechanisms are a potential source of contamination, which is particularly undesirable in an EUV apparatus which must be maintained at a vacuum.
It is an aspect of the present invention to improve local control of the illumination dose.
This and other aspects are achieved according to the invention in a lithographic apparatus including a radiation system constructed and arranged to supply a projection beam of radiation; a support structure constructed and arranged to support a patterning device, the patterning device constructed and arranged to pattern the projection beam according to a desired pattern; a substrate table constructed and arranged to hold a substrate; a projection system constructed and arranged to project the patterned beam onto a target portion of the substrate; a positioning device constructed and arranged to move the substrate relative to the projection system in at least a scanning direction; an intensity adjustment device disposed in the radiation system and comprising a plurality of members which, in use, cast penumbras on an illumination field on the patterning device, the penumbras being substantially symmetric in the scanning direction about a center line of the illumination field.
By ensuring that the penumbras are substantially symmetric in the scanning direction about the center line of the illumination field, the introduction of any telecentricity errors can be avoided or minimized. The effect of absorption of energy from the beam on one side of the illumination field is counterbalanced by a corresponding absorption of energy from the other side of the field. Thus, it is not essential that the penumbras be mirror or rotationally symmetric about the center line of the illumination field, rather that the energy absorbed at a distance y from the center line on one side is equal to the energy absorbed at a distance y from the center line on the other side. It is also not necessary that the penumbra cast by each member be symmetric; the sum of the penumbras cast by two or more members may be symmetric, one member compensating for another. It will be appreciated that if the projection system involves mirrors, then the direction at the patterning device corresponding to the scanning direction of the substrate may not be parallel to the scanning direction. In that case, the symmetry requirement on the penumbras should be understood as being in the direction corresponding to the scanning direction.
It will be appreciated that symmetry may be provided by ensuring the penumbras extend across the illumination field, or by centrally placed penumbras, or by penumbras extending inwards from both sides of the illumination field. Because the effect on telecentricity is greater if radiation is blocked at the edges of the field than in the center, the symmetry requirements are less rigid for penumbras in a central region of the slit.
Preferably, the members of the intensity adjustment device according to the invention are selectively adjusted to manipulate their penumbras on the patterning device. By making the members adjustable, a dynamic adjustment of dose is possible to compensate for changing conditions of the apparatus, illumination settings or pattern being imaged. In an embodiment, the members are blades which are rotated to alter their effective width in a direction perpendicular to the projection beam and thus change the amount of radiation in the projection beam which is intercepted.
The penumbras of the adjustable members can be arranged to extend across the illumination field (slit) on the patterning device (mask) at a slight angle (preferably equal to the pitch of the members divided by the field width) in order that adjustment of their width does not introduce telecentricity problems. Although the intensity of the beam is locally varied by such adjustment, this variation is symmetric across the width of the illumination field. The intensity adjustment device of the invention can also be used for adjustment of dose across the whole illumination field.
The intensity adjusting device of the present invention may be used to correct for non-uniformities in the intensity of the projection beam in the illumination field and variations in the reflectivity or transmissivity of the patterning means. Blades in areas where the intensity of the illumination beam is relatively high, or reflectivity/transmissivity relatively high, are inclined to block a greater proportion of the radiation of the projection beam. The intensity adjusting device can also be used to correct line-width variations caused by other effects by the use of user-defined dose profiles.
In a scanning apparatus, the intensity adjusting device of the invention can be used to effect control over the illumination beam intensity on a fine grid. To do this, the rotational positions of the blades are controlled in synchronism with the scan. Such a control over the illumination beam intensity can be used to compensate for pattern dependent effects, such as near-angle and far-field stray light in dense portions of a pattern or portions with a small non-reflective area on a reflective background. This stray light causes a local increase in background intensity at the substrate which would increase the total dose locally and cause line width variations since the resist is sensitive to total dose received. The intensity adjusting device of the invention can be used to compensate for this.
According to a further aspect of the invention there is provided a device manufacturing method comprising providing a substrate that is at least partially covered by a layer of radiation-sensitive material; providing a projection beam of radiation using a radiation system; using a patterning device to endow the projection beam with a pattern in its cross-section; projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material while moving the substrate in at least a scanning direction; disposing in the projection beam upstream of the patterning device an intensity adjusting device comprising a plurality of members arranged to cast penumbras on an illumination field on the patterning device, the penumbras being substantially symmetric in the scanning direction about a center line of the illumination slit.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. One of ordinary skill in the art will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present application, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet radiation (EUV), (e.g. having a wavelength in the range 5-20 nm).